The present invention relates to an information recording medium and an optical recording/reproducing apparatus, and more particularly to an information recording medium and optical recording/reproducing apparatus for use with a super resolution technology that reproduces pits smaller in size than optical resolution by using heat generated upon laser irradiation.
Optical disks are widely used as information recording media. When an optical disk is used to record a signal or reproduce a recorded signal, an information recording layer of the optical disk is irradiated with laser light that is focused by an objective lens. The size of the resulting focused light spot is expressed by λ/NA where λ is the wavelength of the laser light and NA is the numerical aperture of the objective lens. When this spot size is used to reproduce a repetitive pattern in which data pits and spaces have the same length, the data pit size for acquiring a finite reproduction signal amplitude is λ/4 NA or larger. Sizes smaller than λ/4 NA are said to be smaller than optical resolution. Conventional optical disk technologies represented by CDs, DVDs, HD-DVDs, and Blu-ray Discs (BDs) assume that the minimum size of employed data pits is not smaller than optical resolution. Therefore, the recording capacity of optical disks is increased from 0.65 GB (CDs) to 25 GB (BDs) mainly by decreasing the wavelength λ of laser light from 780 nm to 405 nm and increasing the numerical aperture NA of the objective lens from 0.5 to 0.85, thus reducing the size of the focused light spot.
An optical disk using data pits smaller in size than optical resolution is described in Optical Data Storage 2007, TuB2. As regards this optical disk, only the shortest pit is smaller in size than optical resolution, whereas the others are not smaller in size than optical resolution. As the information on this optical disk is reproduced in the same manner as with the conventional optical disk technology, the signal amplitude derived from the shortest pit is substantially zero. However, pits other than the shortest pit are not smaller in size than optical resolution, and the amplitudes of reproduction signals derived from such pits are finite. Therefore, a signal derived from the shortest pit can also be decoded when a signal process is performed with reference to the signals derived from pits other than the shortest pit. This enables this optical disk to achieve a surface density of 42 GB.
It is conceivable that the capacity can be further increased by decreasing the wavelength λ of an employed light source and increasing the numerical aperture NA of an employed lens. However, when the light source wavelength is shorter than 405 nm, it is the wavelength of ultraviolet light. In such an instance, the light is absorbed by a disk substrate and a protective layer. Thus, it would be difficult to achieve satisfactory recording/reproducing quality. Further, if the lens numerical aperture is increased, near-field light is emitted from the objective lens. This makes it necessary to unduly decrease the distance between the objective lens and a medium during a recording/reproduction process. If such a configuration is employed, a read/write error is likely to occur due to the deformation or dirtiness of an employed disk. Thus, it would be difficult to change the medium although optical disks have an advantage of being readily changeable.
Meanwhile, a super resolution technology is proposed as a different method for achieving high density. The super resolution technology makes it possible to reproduce pits smaller in size than optical resolution (super resolution reproduction) by providing an optical disk with a certain mechanism.
A super resolution technology based on the use of a phase-change material is described, for instance, in Non-patent Document 1. Under normal conditions, the phase-change material is used as a recording film for rewritable optical disks such as CD-RW, DVD-RAM, DVD±RW, and Blu-ray discs. The phase-change material changes its phase (crystalline, molten, or noncrystalline) and optical characteristics in accordance with the heat generated upon laser irradiation. The method disclosed in Non-patent Document 1 uses an optical disk that is produced by forming a film of a phase-change material (a phase-change film) on a read-only type (ROM) substrate. When the information on the optical disk is reproduced, a part of the phase-change film within an irradiation light spot melts due to the heat generated upon laser irradiation so that the optical characteristics (e.g., refractive index and reflectivity) change. When the irradiation light spot on a recording medium contains a region where the optical characteristics are changed, the status of light reflected from the irradiation light spot is different from a case where the irradiation light spot does not contain the region where the optical characteristics are changed. The light reflected when the irradiation light spot contains a region where the optical characteristics are changed changes its status to a greater extent in accordance with a signal of the ROM substrate than when the irradiation light spot does not contain the region where the optical characteristics are changed. Therefore, it is possible to reproduce pits that are smaller in size than optical resolution. As described above, the super resolution technology is a technology for reproducing microscopic pits by using heat that is generated upon laser irradiation for reproduction. Here, a substance that changes its optical characteristics in accordance with temperature and is used to achieve super resolution is called a super resolution substance.
The super resolution technology described in Non-patent Documents 2 and 3 uses a phase-change material as the super resolution substance. This super resolution technology uses a disk constructed so that only a pit (or mark) portion is made of a phase-change material. According to Non-patent Document 2, a disk is prepared by performing crystalline/noncrystalline selective etching on a phase-change film to leave noncrystalline marks only and forming a protective film on a space portion. According to Non-patent Document 3, on the other hand, an optical disk is prepared by chemically polishing a phase-change film formed on a ROM type substrate and embedding a phase-change material in a concaved pit portion only. Super resolution reproduction is accomplished because the phase-change film for pits in a high-temperature region within an irradiation light spot melts upon laser irradiation for reproduction to change the optical characteristics. When this method is used, the phase-change material exists only in a pit portion. Therefore, a melt region can be limited to a smaller size than when the phase-change film is formed on the whole surface. Consequently, the use of this method makes it possible to reproduce extremely small pits.
Technologies based, for instance, on multilayering, near-field light, or two-photon absorption are proposed as optical disk density enhancement technologies that differ from the technologies disclosed in the above non-patent Documents. The multilayering technology increases the capacity in the cubic direction by providing a disk with a plurality of information recording layers, which are more or less spaced apart from each other. Recording and reproducing operations for each layer are performed by focusing a laser beam on each layer. As mentioned in Non-patent Document 2, it is expected that the multilayering technology can be combined with the super resolution technology.